substances is then numerically expressed by stating the resistance of
one cubic centimetre of the substance taken between opposed faces, and
expressed in ohms, microhms or megohms, as may be most convenient. The
reciprocal of the ohm is called the mho, which is the unit of
conductivity, and is defined as the conductivity of a substance whose
resistance is one ohm. The absolute unit of conductivity is the
conductivity of a substance whose resistivity is one absolute C.G.S.
unit, or one-thousandth-millionth part of an ohm. Resistivity is a
quality in which material substances differ very widely. The metals
and alloys, broadly speaking, are good conductors, and their
resistivity is conveniently expressed in microhms per cubic
centimetre, or in absolute C.G.S. units. Very small differences in
density and in chemical purity make, however, immense differences in
electric resistivity; hence the values given by different
experimentalists for the resistivity of known metals differ to a
considerable extent.

I. CONDUCTION IN SOLIDS

It is found convenient to express the resistivity of metals in two
different ways: (1) We may state the resistivity of one cubic centimetre
of the material in microhms or absolute units taken between opposed
faces. This is called the _volume-resistivity_; (2) we may express the
resistivity by stating the resistance in ohms offered by a wire of the
material in question of uniform cross-section one metre in length, and
one gramme in weight. This numerical measure of the resistivity is
called the _mass-resistivity_. The mass-resistivity of a body is
connected with its volume-resistivity and the density of the material in
the following manner:--The mass-resistivity, expressed in microhms per
metre-gramme, divided by 10 times the density is numerically equal to
the volume-resistivity per centimetre-cube in absolute C.G.S. units. The
mass-resistivity per metre-gramme can always be obtained by measuring
the resistance and the mass of any wire of uniform cross-section of
which the length is known, and if the density of the substance is then
measured, the volume-resistivity can be immediately calculated.

If R is the resistance in ohms of a wire of length l, uniform
cross-section s, and density d, then taking [rho] for the
volume-resistivity we have 10^9R = [rho]l/s; but lsd = M, where M is
the mass of the wire. Hence 10^9R = [rho]dl^2/M. If l = 100 and M = 1,
then